Plasma-spraying device

ABSTRACT

Example embodiments relate to a plasma-spraying device for spraying a powdered material, including electrodes, which may form a plasma channel having an inlet end and an outlet end, and a unit for supplying the powdered material to the plasma channel. The powder supply unit may be arranged between a first section of the electrodes located upstream of the powder supply unit and a second section of the electrodes located downstream of the powder supply, as seen in the direction of plasma flow of the plasma channel.

FIELD OF THE INVENTION

The present invention relates to a plasma-spraying device for spraying apowdered material, comprising electrodes, which form a plasma channelhaving an inlet end and an outlet end, and a means for supplying saidpowdered material to said plasma channel. The invention further concernsa method of plasma-spraying. Finally, the invention also concerns theuse of a plasma-spraying device for incinerating a powdered material.

BACKGROUND ART

Plasma-spraying devices or plasmatrons are used for low-power thermalspraying of powdered materials, for example in connection with differentkinds of surface-coating. Such devices generally comprise a cathode, ananode and a plasma channel formed therebetween. During use of the devicean electric arc is generated in the plasma channel, between the anodeand the cathode, and gas is then introduced in the plasma channel forforming a plasma. The plasma jet thus flows through the plasma channelfrom an inlet end adjacent the cathode to an outlet end adjacent theanode. At the same time, a powdered material is supplied to the plasmajet for spraying thereof.

Today, one of two alternatives is used for supplying the powder.

According to the first alternative, the powder is introduced in theoutlet area of the plasma channel, adjacent the anode. One advantage ofthis alternative is that when the powder is supplied the plasma flow isfully developed and has certain determined properties (temperature,velocity, sectional area, energy, etc.). These properties are dependent,inter alia, on the geometry of the plasma channel, the plasma-generatinggas used and the amount of energy supplied. A further advantage ofsupplying the powder at the anode is that the heating of the plasma flowis not affected by the properties of the powdered material.

In connection with this variant of powder supply the powder is usuallysupplied perpendicularly to the plasma flow. The path of the powderparticles travelling out from the anode area towards the surface to becoated will thus depend largely on the size and weight of the particles.The larger and heavier particles enter the high-temperature zone of theplasma jet directly, whereas the lighter ones first reach the centre ofthe plasma jet only in relatively cold zones located relatively far awayfrom the anode. This means that there is a risk of part of the powderparticles not being sufficiently hot and, moreover, of them missing thetarget, i.e. the object to be coated, for example, with the powderedmaterial.

This is disadvantageous in that a large part of the powdered material iswasted, resulting in poor material economy. In other words, thepowder-sprayed coating is produced using only a small part of the powdersupplied. This is particularly disturbing when expensive coatingmaterials are being used. The problem can be solved to some extent byusing more homogeneous powders. A disadvantage associated with suchpowders, however, is that they are difficult to manufacture and, thus,relatively expensive.

To avoid problems associated with a perpendicular powder supply at theoutlet area of the plasma channel, attempts have been made to provide asupply pipe for horizontal powder supply, which pipe is arrangeddirectly in the plasma jet. However, one disadvantage hereof is thatproblems arise in connection with the heating of the plasma flow andthat the plasma flow properties are greatly interfered with.

A further disadvantage generally associated with the introduction of thepowdered material in the anode area, at the outlet of the plasmachannel, is that a large amount of energy is needed to maintain the hightemperature and specific power (power per unit of volume) of the plasmaflow, so as to obtain, in turn, a homogeneous coating. It is believedthat this is due to the fact that the plasma flow at the outlet of theplasma-spraying device, where the powder is supplied, has a virtuallyparabolic temperature and velocity distribution. Thus, the temperatureand velocity gradient and the thermal enthalpy of the plasma flow areinversely proportional to the diameter of the plasma jet. To increasethe homogeneity of the spray coating it is therefore necessary toincrease the diameter of the plasma jet, which requires a lot of energy.

U.S. Pat. Nos. 3,145,287 and 4,445,021 discloses plasma-spraying devicesin which the powdered material is introduced in the anode area, at theoutlet of the plasma channel.

According to a second known alternative, the powder is supplied at theinlet of the plasma channel, at the cathode. In this case, the powder isheated by the electric arc simultaneously with the plasma-generatinggas. The cathode area is considered to be a cold zone, which allows thepowder to be introduced in the centre of the plasma flow.

When supplying gas at the cathode area in a plasma channel where anelectric arc is generated at a predetermined discharge current, a smallpart of the gas will flow into the central part of the channel where thetemperature is high, while the remaining part of the gas will flow alongthe channel walls, forming a cold gas layer between the channel wallsand the electric arc. By using this gas distribution only a small partof the powder supplied at the inlet will flow into the electric arc,while the large part of the powder will flow in the cold layer adjacentthe channel walls. This results in the powder being unevenly heated andthe process being difficult to control. Furthermore, the channel and theanode risk being clogged by the powder, which thus has a detrimentaleffect on the conditions required for a stable plasma flow.

Trying to increase the transfer of mass to the central part of thechannel by increasing the gas and powder flow is not a practicablealternative. The reason is that if the gas and powder flow is increased,while the current is kept constant, the diameter of the electric arcwill decrease, which aggravates the problem of powder materialaccumulating in the cold areas along the channel walls. At the sametime, the time during which the powder particles that actually end up inthe heating zone remain in this zone decreases, since their velocityincreases. This further reduces the quality of the process. Therefore,the amount of material in the hot zone cannot be increased if thecurrent remains constant. Increasing the current implies, in turn,disadvantages both for the design and handling of the plasma-sprayingdevice.

U.S. Pat. Nos. 5,225,652, 5,332,885 and 5,406,046 discloseplasma-spraying devices in which the powder is supplied at the cathode.

When analysing plasma-spraying processes, it has been found that theproperties of the coating formed mainly depend on the thermal conditionand velocity of the powder during spraying. The term “thermal condition”here primarily means the thermal profile and state of aggregation of thematerial. In prior-art plasma-spraying devices it is difficult, asdescribed above, to control the thermal condition and velocity of thepowder.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improvedplasma-spraying device for low-power spraying of powdered materials,which allows satisfactory control of the coating properties as well asgood homogeneity. Moreover, the invention shall allow spraying ofcoatings of materials and compounds with different properties. Finally,it shall also be possible to use the invention for breaking downpowdered materials.

According to the invention this object is achieved by means of a deviceof the kind stated by way of introduction, in which said powder supplymeans is arranged between a first section of said electrodes locatedupstream of the means and a second section of said electrodes locateddownstream of the means, as seen in the direction of plasma flow of theplasma channel from inlet end to outlet end.

Thus, the powdered material is supplied neither at the inlet end(cathode end) nor at the outlet end (anode end) of the plasma channel,but somewhere along the channel, between two sections thereof. Owing tothis design, it is possible to control the properties of the plasma flowboth before and after the powder has been supplied to the plasma flowand, thus, to control the velocity and heat of the powder particles insuch manner that the desired coating properties and good homogeneity canbe obtained. Furthermore, the plasma-spraying device according to theinvention allows the use of a plasma channel with a relatively smalldiameter, which results in a low power consumption and low operatingcurrents.

The section located upstream of the powder supply can then suitably beused to create the optimal conditions in the plasma flow, so that thematerial is heated effectively. The section located downstream of thepowder supply allows control of the heating of the powdered material andother characteristics of the powder, such as its velocity. In thismanner, high efficiency and satisfactory control of the plasma-sprayingprocess can be obtained.

Preferably, the section located upstream of the powder supply means andthe section located downstream of the powder supply means can bedesigned in such manner that they, when the plasma-spraying device isbeing used, bring about different conditions in the plasma channel.

The first section (upstream of the powder supply) is adapted to heat theplasma flow and its characteristics are such that it can provideefficient and fast heating of the powder across the sectional area ofthe channel. Preferably, the total length of all electrodes in thesection is enough for the gas to be fully heated, i.e. for the desiredtemperature profile to be obtained. This significantly reduces theamount of powder that might otherwise stick to the channel walls due tothe fact that it is sufficiently heated.

At the second section (downstream of the powder supply) additionalenergy is supplied initially to compensate for the cooling of the plasmathat occurs since the powder is usually introduced in the channeltogether with a cold carrier gas. Furthermore, the energy supply iscontrolled at the second section so that the desired properties of theplasma jet are obtained and also so that the powder reaches the velocityand heat level necessary to obtain the required adhesion, structure andporosity in the spray coating.

Preferably, the sections can be caused to bring about differentconditions in the plasma channel by at least one of the followingparameters differing between said first and second sections: the lengthof the section, the number of electrodes in the section and the geometryof the plasma channel in the section.

Suitably, a plurality of powder supply means can be provided, each ofsaid powder supply means being arranged between a section of saidelectrodes located upstream of the means and a section of saidelectrodes located downstream of the means. This is particularlyconvenient when more than one kind of powder is supplied forspray-coating purposes. Thus, each powder sort can be suppliedseparately and the different powder sorts do not have to be mixed up,which ensures the desired ratio between the different powder sorts interms of the amount supplied.

The number of electrodes in a section can be no less than one. However,the number of electrodes in at least one section is preferably two. Thisis advantageous for the following reasons: The discharge current in thechannel portion of each section has the same value. The centre of theelectric arc, along the centre axis of the plasma channel, has atemperature T that is proportional to the ratio of the discharge currentI to the diameter d (T=(I/d)) of the plasma channel. Consequently, toincrease the temperature level in the plasma flow at the end of asection while maintaining a low current level, the cross section of theplasma flow, and thus the cross section of the electric arc that heatsthe gas, must be reduced. If the cross section of the electric arc issmall the electric field strength in the channel has a high value andthe voltage of the section can be several times greater than the naturalvoltage of the plasma for the commonly used types of plasma-generatinggas.

If, at the same time, it is necessary to heat a relatively large gasflow in order to effectively heat the powder that is introduced afterthis section, then the channel must have a relatively great length. Thereason for this is that, to reach the same temperature as that at thecentre of the electric arc, the heated gas flow must pass a certainlength of the plasma channel along the centre axis of the plasmachannel, which length corresponds to the heating distance of the gas. Ifthe gas flow increases so does the heating distance of the gas, whichmeans that the length of the plasma channel in the section must berelatively great.

The combination of a small cross section of the channel and a greatlength thereof in the section thus results in a high field strength overa relatively great distance, which means that, instead of one longelectric arc, two shorter, consecutive arcs can be generated. Theseshorter arcs burn at a lower voltage and do not heat the gas effectivelyto a high temperature. The problem of dividing the electric arc intoshorter arcs is prevented by dividing the section into at least twoseparate electrodes that are electrically insulated relative to oneanother. The number of electrodes, as well as the length of eachelectrode, depend on the desired gas flow level and the gas jettemperature at the end of the section. Thus, the plasma device can beformed with a relatively small diameter of the plasma channel, whichresults in a low power consumption and low operating currents. Thisallows low-power spraying to be obtained.

In certain applications it is particularly convenient for the number ofelectrodes in the section closest to the inlet end of the plasma channelto be at least two, so as to reduce the risk of the electric arc beingdivided into two shorter electric arcs.

For supplying powder to the plasma channel the powder supply meanssuitably forms a space that makes an angle of less than 90° with acentre axis of the plasma channel. Suitably, said space can be formed bya projection on the electrode closest upstream of the means, which isarranged at a distance from a recess on the electrode closest downstreamof the means.

By inserting the powder at an angle smaller than 90° relative to acentre axis of the plasma channel the powder can be conveyed to thecentre of the plasma and there is less risk of it adhering to thechannel walls.

Preferably, said projection is conical and forms an angle (α) with thecentre axis of the plasma channel, which angle (α) is suitably in therange of 15-25°. Said recess can thus suitably be conical and forms anangle (β) with the centre axis of the plasma channel, which angle (β) ispreferably in the range of 17-30°. In this connection, the projection isconveniently arranged at a distance from the recess, in such manner thatit is partly inserted therein, whereby the space for introducing powderat an angle to the centre axis of the plasma channel is formed betweenthe projection and the recess. Said space gets a particularly convenientshape if the difference between said angle of the recess and said angleof the projection (β-α) is 1.5° to 5°.

In this way, the powder is introduced in a satisfactory manner in thedischarge channel, essentially along its centre line.

Depending on the kind of powder used, it may be introduced through acircular, ring-shaped opening, through a system of holes or tangentiallyto the cross section of the channel. Tangential insertion causesvortices to occur, which is particularly desirable for certain types ofpowder.

Suitably, the diameter of the plasma channel in at least one section isgreater than the diameter of the plasma channel in the section locatedupstream of said section. Preferably, the channel diameter ofconsecutive sections increases, so that the diameter of the plasmachannel in one section is greater than the diameter of the plasmachannel in every section located upstream of said section. This isadvantageous since each time powder and carrier gas are supplied theflow through the plasma channel increases. To prevent the velocity inthe channel from increasing with the increased flow, which would reducethe heating time for the plasma and the powder, it is thereforeconvenient to increase the diameter of the plasma channel.

As the greatest electric field strength is produced at the cathode, thelength of the electrodes is suitably increased by the distance from thecathode, since the field strength decreases with the distance from theinlet end of the plasma channel. Thus, initially the electrode length ispreferably small and increases towards the end of the section.Preferably, at least in one section, the length of the furthest upstreamelectrode equals the diameter of the plasma channel at said electrodelocated furthest upstream. Suitably, all electrode lengths can bedetermined by the formula ln=n× dchannel, where in is the length ofelectrode n and n is the ordinal number of the electrode in a section,as seen from the inlet end of the plasma channel. dchannel is thechannel diameter of electrode n (l1 is the length of the electrodeclosest to the inlet end of the plasma channel, whose length equals itsdiameter, l1=1× dchannel).

Suitably, the plasma channel is formed by annular electrodes, whichadvantageously can be coaxially arranged.

The invention further concerns a method of plasma-spraying a powderedmaterial by using a plasma-spraying device comprising electrodes, whichform a plasma channel having an inlet end and an outlet end. In themethod according to the invention, the powdered material is supplied tothe plasma-spraying device in at least one supply point located betweentwo sections of said electrodes, which sections are located respectivelyupstream and downstream of the supply point.

The advantages of the present invention over prior art correspond to theones described above in connection with the device.

Preferably, the section located upstream of the supply point is used tobring about the necessary conditions in the plasma flow. Furthermore,the section located downstream of the supply point is suitably used tocontrol the heating of the powdered material and other properties of thepowder.

Finally, the invention concerns the use of a device according to theinvention for incinerating a powdered material. When incinerating apowdered material, the material is supplied to the device, in which theplasma is used to incinerate the powdered material or transform it intonew substances. This is used in particular to incinerate or transformmaterials that are harmful to the environment or otherwise harmfulmaterials.

In this kind of incineration, besides the powdered material to beincinerated, additional powdered material may conveniently be suppliedfor neutralisation or transformation of the powdered material intendedto be incinerated. Suitably, the additional material is supplied througha material supply means other than the one used for the material to beincinerated.

The excellent possibilities for influencing the characteristics in theplasma channel of the device according to the present invention makes itparticularly suitable for incinerating various types of material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below with reference tothe accompanying schematic drawings, which by way of example illustratecurrently preferred embodiments of the invention.

FIG. 1 is a sectional view of a first embodiment of a plasma-sprayingdevice according to the invention with two powder supply means.

FIG. 2 is a sectional view of the embodiment in FIG. 1 along the lineII-II.

FIG. 3 is a sectional view of a second embodiment of a plasma-sprayingdevice according to the invention, in which the cross section of thechannel increases for each section with the distance from the cathode.

FIGS. 4 a and 4 b illustrate two variants of the supply means along thesection IV-IV in FIG. 1.

FIG. 5 illustrates a third variant of the supply means along the sectionV-V in FIG. 1.

FIG. 6 is a cross-sectional view along the line VI-VI in FIG. 2.

FIG. 7 illustrates a portion of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates one embodiment of a plasma-spraying device accordingto the invention comprising a cathode 14, preferably made of tungstencontaining lanthanum, which is arranged in a cathode holder 16. Thecathode holder 16 has an internal channel 17 which acts as a means forsupplying plasma-generating gas G and as a cooler for the cathode holder16. The device further comprises a number of coaxially arranged annularelectrodes 1, which form a plasma channel 2. The plasma channel 2extends from the cathode 14 at its inlet end 3 to an anode 15 at itsoutlet end 4. In use, an electric arc is generated in the device betweenthe cathode 14 and the anode 15, which arc heats the plasma-generatinggas to form a plasma. The internal channel 17 of the cathode holder thusopens into the inlet end 3 of the plasma channel, from where plasma willflow through the channel to the outlet end 4 of the plasma channellocated adjacent the anode 15, where it is discharged.

A first means 5 for supplying a first powdered material PG1 is arrangedbetween a first section 6 of electrodes 1 located upstream of the supplymeans 5 and a second section 7 of electrodes 1 located downstream of thesupply means 5. Furthermore, a second means 9 for supplying a secondpowdered material PG2 is arranged between said second section 7 and asection 8 of electrodes 1 located downstream thereof.

The first section 6 is used to heat the plasma-generating gas G, whichis supplied through the channel 17. The number of electrodes in thissection 6 is determined on the basis of the desired heating of the gasflow; it here comprises three electrodes 1.

The second section 7 is used partly to influence the plasma-generatinggas in a suitable manner prior to the introduction of the secondpowdered material PG2, partly to give the first powdered material PG1suitable characteristics. The second section 7 here comprises threeelectrodes 1.

Finally, the third and, in this case, last section 8 is used to giveboth the powdered materials PG1 and PG2 suitable properties for sprayingon a surface to be coated from the anode 15 of the plasma sprayingdevice. In this case, the third section 8 comprises three electrodes 1as well.

Thus, in this case there are at least two electrodes 1 in each section6, 7, 8, which reduces the risk of double arcs being generated in thesection.

The powdered materials PG1 and PG2 are suitably supplied throughrespectively the first 5 and the second 9 powder supply means each bymeans of a stream of cold carrier gas through respectively a first 18and a second 19 supply pipe. The powder supply means 5, 9 are preferablydesigned in such manner that the last, furthest downstream electrode 1in the section 6 located upstream of the means has a projection 11,which here is conical and forms an angle α with the centre axis of theplasma channel (see FIG. 7). The first, furthest upstream electrode 1 inthe section 7 located downstream of the means 5 has a recess 12, whichhere is conical and forms an angle β (see FIG. 7) with the centre axisof the channel. Suitable angles are 15-25° for α and 17-30° for β. Theterm conical is used here in the general sense; as shown in FIG. 1 theshape is that of a truncate cone. This shape facilitates an even supplyof the powder to the plasma flow.

The projection 11 is partly inserted in the recess 12, but arranged atsuch a distance therefrom that a powder supply space 10 is formedbetween the projection 11 and the recess 12, which space 10 forms anangle with the centre axis of the plasma channel 2.

An expansion chamber 20 is provided which is connected with the space 10associated with the first supply means 5 and to which the powdermaterial PG1 and its carrier gas are supplied. The powder is introducedin the plasma channel through openings 13 (see FIG. 4 a). An evendistribution of the powder in the channel is here obtained by supplyingthe powder-transporting gas through the openings 13, which form groovesoriented at an angle to radii of the plasma channel 2. This type ofsupply is here called tangential supply, since it takes placetangentially to the cross section of the channel, and is used to createvortices in the powder when it is introduced into the channel 2.According to a second embodiment (FIG. 4 b), powder-transporting gas issupplied to the plasma channel 2 via a small, circular, ring-shapedopening 13′.

Similarly, an expansion chamber 21 is provided which is connected withthe other space associated with the second supply means 9. In this case,powder-transporting gas is supplied through a system of evenlydistributed holes 13″ in a circle, which are drawn along radii of theplasma channel 2 (FIG. 5).

It goes without saying that the formation of openings 13 according toone of the embodiments shown in FIGS. 4 a, 4 b and 5 can be variedbetween the different supply means 5, 9, as required.

More specifically, the plasma-spraying device comprises a conductivecylindrical body 22, on which an anode 15 is arranged by means of aconductive washer 23 and nut 24. The body 22 contains a dielectriccasing 25. The cathode holder 16 and the first electrode 1 in the firstsection 6 are arranged in a second dielectric casing 26. To protect thecasing 26 from the heat a ceramic casing 27 is used. To cool theplasma-spraying device the body 22 has channels 28 (see FIG. 2) throughwhich a coolant W is supplied to the anode 15. On the way the electrodes1 are also cooled. The electrodes 1 are interconnected by means ofinsulated, watertight gaskets 29. In addition, an anode seal 30 isprovided, which may be of the same material as that used for thewatertight gaskets 29. A water- and gastight seal is ensured at themoving contact surfaces by means of sealing rings 31, 32, 33. Thesealing force is obtained by means of screws 34 and a washer 35. Thescrews 34 are further connected to the positive pole of the power sourceof the plasma-spraying device. The negative pole of the power source isconnected to the cathode holder 16. The main part of theplasma-generating gas G is supplied through the channel 17 in thecathode holder 16. Powder and powder-transporting gas are suppliedthrough supply pipes 18, 19 to the respective powder supply means 5, 9.

When using the embodiment of the device shown in FIG. 1plasma-generating gas G is first introduced in the plasma-sprayingdevice through the channel 17 to the plasma channel 2. At the same timea coolant W is supplied through the cooling channels 28 to ensurecooling of the plasma-spraying device. A high voltage triggering systemis then switched on, which initiates a discharge process in the plasmachannel 2 of the plasma-spraying device and ignites an electric arcbetween the cathode 14 and the anode 15. Transporting gas PG1 and PG2 isthen supplied through the supply pipes 18, 19, following which thepowder supply is initiated through the supply means 5, 9.

To switch off the device, the supply of powder is first turned off. Theoperating current is then turned off and, after a certain time, thesupply of the transporting gas and the plasma-generating gas is stoppedand, finally, the cooling system is turned off.

When optimal conditions apply it is possible to use the same powersource for a set of different plasma-spraying devices which are used forplasma-spraying a plurality of different coatings, such as ceramics,materials with high melting point, materials with low melting point,wear-resistant materials, etc. If argon is used as plasma-generating gasit is suitable for the power source to have a stable operating currentof 10-40 A when the operating voltage of the plasma-spraying device is40-80 V. The operating voltage of the plasma-spraying device depends onthe number of sections and the lengths thereof. At a gas consumption of1-4 l/min and a heating temperature of 8000-12000° C. the channels havea diameter of preferably 1-2 mm. The effect of the plasma flow at theend of the first section at this temperature level is determined by thelength of the section, and to eliminate the risk of a double electricarc being created the number of electrodes in the section should be noless than two.

FIG. 3 shows a further embodiment of a plasma-spraying device accordingto the invention. The parts thereof that have equivalents in theembodiment initially described, illustrated in FIG. 1, have beenprovided with the corresponding reference numerals, and for adescription thereof reference is made to the above description of thefirst embodiment.

The embodiment shown in FIG. 3 differs from the embodiment shown in FIG.1 as regards the geometry of the plasma channel 2. In this case, thediameter of the plasma channel 2 increases with every section 6, 7, 8,i.e. in such manner that the every consecutive section has a greaterdiameter than the previous section. This design reduces the risk of thepowder material sticking to the inner walls of the plasma channel.Preferably, the diameter here increases according to the formula statedabove.

In general, the diameter of the channel greatly influences the velocityof the powder particles. Since the properties of the formed coatingslargely depend on the velocity when contact is made with the surface tobe coated, the channel diameter can conveniently be varied to obtain thedesired effect. Another property that greatly influences the propertiesof the formed coatings is the temperature of the powder, which likewise,as described above, can be appropriately regulated in the deviceaccording to the invention. To sum up, it is possible to control boththese properties by choosing suitable parameters, such as length andchannel diameter of the section located upstream of the powder supplyand the section located downstream of the powder supply.

It will be appreciated that a number of modifications of the embodimentdescribed above are conceivable within the scope of the invention, asdefined by the appended claims. As described above, for example, eachsection may thus instead comprise two or more than three electrodes.Furthermore, it is not necessary to have the same number of electrodesin each section. Finally, the geometry of the plasma channel may vary.

1. A plasma-spraying device for spraying a powdered material, comprisingelectrodes, which form a plasma channel having an inlet end and anoutlet end, and means for supplying said powdered material to saidplasma channel, wherein said powder supply means is arranged between afirst section of said electrodes located upstream of the means and asecond section of said electrodes located downstream of the means, asseen in the direction of plasma flow of the plasma channel, and whereinthe diameter of the plasma channel in at least one section is greaterthan the diameter of the plasma channel in each section located upstreamof said section.
 2. A plasma-spraying device as claimed in claim 1,wherein at least one of the following parameters is different betweensaid first and second sections: the length of the section, the number ofelectrodes in the section and the geometry of the plasma channel in thesection.
 3. A plasma-spraying device as claimed in claim 1, wherein anadditional powder supply means is arranged between a third section ofelectrodes and one of said first and second sections.
 4. Aplasma-spraying device as claimed in claim 1, wherein a plurality ofpowder supply means are provided, each of said powder supply means beingarranged between a section of said electrodes located upstream of themeans and a section of said electrodes located downstream of the means.5. A plasma-spraying device as claimed in claim 1, wherein the number ofelectrodes in at least one section is at least two.
 6. A plasma-sprayingdevice as claimed in claim 5, wherein the number of electrodes in thesection closest to said inlet end of the plasma channel is at least two.7. A plasma-spraying device as claimed in claim 1, wherein the powdersupply means forms a space for supplying powder at an angle to a centeraxis of the plasma channel.
 8. A plasma-spraying device as claimed inclaim 7, wherein said space is formed by a projection on the electrodeclosest upstream of the means, which is arranged at a distance from arecess in the electrode closest downstream of the means.
 9. Aplasma-spraying device as claimed in claim 8, wherein said projection isconical and makes an angle (α) with the center axis of the plasmachannel.
 10. A plasma-spraying device as claimed in claim 9, whereinsaid angle (α) is 15-25°.
 11. A plasma-spraying device as claimed inclaim 8, wherein said recess is conical and makes an angle (β) with thecenter axis of the plasma channel.
 12. A plasma-spraying device asclaimed in claim 11, wherein said angle (β) is 17-30°.
 13. Aplasma-spraying device as claimed in claim 11, wherein the differencebetween said angle of the recess and said angle of the projection (β-α)is 1.5° to 5°.
 14. A plasma-spraying device as claimed in claim 1,wherein the powder supply means comprises openings that are oriented atan angle to the center axis of the plasma channel to obtain a tangentialpowder supply.
 15. A plasma-spraying device as claimed in claim 1,wherein the diameter of the plasma channel in one section is greaterthan the diameter of the plasma channel in the section located upstreamof said section.
 16. A plasma-spraying device as claimed in claim 1,wherein the length of the electrodes is increased by their distance fromthe inlet end of the plasma channel.
 17. A plasma-spraying device asclaimed in claim 1, wherein at least in one section, the length of thefurthest upstream electrode equals the diameter of the plasma channel insaid furthest upstream electrode in said section.
 18. A plasma-sprayingdevice as claimed in claim 1, wherein at least in one section, thediameter of the plasma channel varies in said section.
 19. Aplasma-spraying device as claimed in claim 1, which further comprises acathode and an anode arranged at a distance from the cathode and coaxialtherewith, between which an electric arc is generated, during use ofsaid device, into which gas is introduced to form a plasma, saidelectrodes being arranged between said cathode and said anode formingsaid plasma channel.
 20. A plasma-spraying device as claimed in claim 1,wherein said electrodes are annular.
 21. A plasma-spraying device asclaimed in claim 1, wherein said electrodes are coaxially arranged. 22.A plasma-spraying device for spraying a powdered material, comprisingelectrodes, which form a plasma channel having an inlet end and anoutlet end, and means for supplying said powdered material to saidplasma channel, wherein said powder supply means is arranged between afirst section of said electrodes located upstream of the means and asecond section of said electrodes located downstream of the means, asseen in the direction of plasma flow of the plasma channel, and whereinat least in one section, the length of the furthest upstream electrodeequals the diameter of the plasma channel in this electrode.
 23. Aplasma-spraying device as claimed in claim 22, wherein the diameter ofthe plasma channel in at least one section is greater than the diameterof the plasma channel in each section located upstream of said section.24. A plasma-spraying device as claimed in claim 22, wherein in onesection, the length of the electrodes in the section, which are locateddownstream of said furthest upstream electrode, is calculated asLn=n×dchannel where Ln is the length of electrode n, n is the ordinalnumber of the electrode in a section and dchannel is the diameter of theplasma channel in said electrode n.
 25. A method of supplying a powderedmaterial by using a plasma-spraying device comprising electrodes, whichform a plasma channel having an inlet end and an outlet end, comprising:supplying the powdered material to the plasma-spraying device in atleast one supply point located between two sections of said electrodes,which sections are located respectively upstream and downstream of thesupply point, wherein the diameter of the plasma channel is adapted inat least one section to be greater than the diameter of the plasmachannel in each section located upstream of said section.
 26. A methodof supplying a powdered material as claimed in claim 25, wherein thesection located downstream of the supply point is used to control theheating of the powdered material and other properties of the powder. 27.A method of supplying a powdered material as claimed in claim 25,wherein at least one of the following parameters is different betweensaid sections located respectively upstream and downstream: the lengthof the section, the number of electrodes in the section and the geometryof the plasma channel in the section.
 28. A method as claimed in claim25, wherein a powdered material is supplied in at least two supplypoints located between the two sections of said electrodes, whichsections are located respectively upstream and downstream of therespective supply points.
 29. A method of supplying a powdered materialby using a plasma-spraying device comprising electrodes, which form aplasma channel having an inlet end and an outlet end, comprising:supplying the powdered material to the plasma-spraying device in atleast one supply point located between two sections of said electrodes,which sections are located respectively upstream and downstream of thesupply point, wherein at least in one section, the length of thefurthest upstream electrode is ad to equals the diameter of the plasmachannel in this electrode.